This invention relates to materials for electrical contacts and electrodes. In particular, this invention relates to tungsten-copper composites.
Alloys for electrical contacts and electrodes are metallurgical composites based on heterogeneous systems (pseudoalloys) of two or more components with vastly different thermophysical properties. The properties of these alloys represent optimal combinations of the component characteristics and are required for operation in applications such as high-current interrupters with gas or oil as arc quenching media, electrical discharge machining, spot welding, and other applications employing an electrical discharge. Common to these applications is the electrical arcing which occurs between the contacts or between an electrode and a workpiece. For example, electrical contacts serve as points of arc attachment at current switching. Despite the momentary duration of switching, electric arcs in oil- or gas-filled high-current interrupters fully develop into high temperature plasma discharges. At the points of arc attachment, plasma discharges generate electrodynamic forces and thermal fluxes causing erosive wear of electrical contacts. Material erosion reaches its peak in electrical contacts operating in an oxidizing environment (e.g., air-blast high-current interrupters). To withstand this wear, electrical contact materials must possess specific thermophysical properties.
Tungsten-copper composites are preferred materials for these applications. However, electrical contacts made from unalloyed Wxe2x80x94Cu composites are prone to cracking in electrical discharge environments. The problem appears to stem from the poor thermal shock resistance of the composite. When the structural continuity is partially or totally disrupted by the loss of copper due to arc heating, the composite loses the ability to undergo plastic deformation as a single structure. If excessive thermal fluxes produced by the arc attachment to the contact are not dissipated quickly, the thermal shock generates high thermal stresses and cracks in the contact. For composites with an average tungsten grain size of about 20 xcexcm, cracking occurs after a period of more or less even ablation. Tungsten-copper composites with an average tungsten grain size of about 5 xcexcm show greater cracking, apparently resulting from further sintering of the tungsten which causes significant shrinkage and pore formation. The erosion rate and cracking can increase considerably if the pore volume in the composite material exceeds approximately 4%.
In addition to thermal shock resistance, the tungsten-copper materials used in these applications should possess resistance to loss of copper, resistance to erosion, and resistance to corrosion. Conventional solutions to these requirements include alloying with 4-5 weight percent (wt. %) Ni and maintaining a low pore volume.
Two basic powder metallurgical (P/M) techniques are used to make Wxe2x80x94Cuxe2x80x94Ni alloys for electrical contacts and electrodes: sintering/infiltration and direct sintering. Sintering/infiltration is a two-step manufacturing process which consists of (1) pressing elemental W powder and sintering the compact using one thermal cycle to form a refractory component skeleton (or framework) with controlled porosity, and (2) infiltrating the skeleton with the electrically/thermally conductive Cu component using another thermal cycle. The sintering/infiltration technique does not allow fabrication of net-shape components and the use of fine W powders (FSSS less than 5 xcexcm). In particular, fine W powders promote localized densification in the W skeleton resulting in partially closed porosity which cannot be infiltrated with Cu. High-temperature sintering (above 1450-1500xc2x0 C.) of W in the presence of liquid Ni promotes the growth of W grains in the skeleton. Brittle Wxe2x80x94Ni intermetallics form along the W grain boundaries during high-temperature sintering of W in the absence of Cu. This degrades the mechanical properties of the W skeleton. In addition, it is difficult to assure uniformly distributed contacts between the W and Ni particles when Ni is used as a sintering aid for W powder.
Conventional direct sintering consists of blending and compacting of the W, Cu, and Ni powders with an average particle size of about 5 xcexcm. Then, depending on the Cu content, the compacts are sintered at temperatures above or below the melting point of Cu. Conventional direct sintering approaches suffer from an inability to separately sinter a strong W skeleton to act as the alloy backbone. In addition, there are problems with (1) excessive coalescence and solid-state sintering of Cu prior to melting, (2) excessive coagulation of Cu at melting, (3) Cu bleedout from an improperly sintered W skeleton, (4) development of excessive porosity ( greater than 4%), (5) disintegration of the W skeleton, and (6) loss of shape (slumping).
Enhanced sintering of Wxe2x80x94Cu is strongly influenced by the formation of a Cu-based liquid phase above 1083xc2x0 C. Ni and Cu have an unlimited mutual solubility which in combination with partial solubility of W in Ni (38 wt. % of W in Ni at 1100xc2x0 C.) greatly improves the wetting of W by Cu and eliminates Cu bleedout. The sintered density, strength, and microhardness increase linearly with Ni additions. The affinity of Ni for both Cu and W introduces a solution-reprecipitation mechanism for sintering W. Operation of this mechanism reaches a significant level at Ni concentrations in the alloy of at least 2 wt. %.
The Cuxe2x80x94Ni liquid phase is chemically active with respect to W. It begins dissolving W and forming a Cuxe2x80x94Nixe2x80x94W matrix. Due to the limited solubility of W in Ni, the concentration of dissolved W in the matrix eventually reaches an equilibrium level. Formation of the Cuxe2x80x94Nixe2x80x94W matrix turns on the solution-reprecipitation mechanism which affects the sintering of W. The matrix acts as a W carrier by dissolving tiny W particles and necks, and transporting and redepositing W onto surfaces of larger particles causing their further growth. This thermodynamically warranted process is governed by kinetic parameters such as the concentration of Ni in the matrix, the size of W particles, and the temperature. Microstructure and mechanical properties of Wxe2x80x94Cuxe2x80x94Ni alloys produced by a solution-reprecipitation mechanism are strongly influenced by two metallurgical phenomena, the Kirkendall effect and Ostwald ripening. Higher diffusion rates of Cu and W in Ni, compared to those for Ni in Cu and W, result in formation of pores and voids (the Kirkendall effect) which may not be totally eliminated by sintering. Coarsening and spheroidization of W particles in the presence of an active Cuxe2x80x94Ni liquid phase (Ostwald ripening) may lead to porosity, disintegration of the W skeleton, and loss of shape (slumping) of the sintered material. Due to the above effects, alloys made by a solution-reprecipitation sintering technique are very sensitive to processing conditions. Even slight changes in sintering temperature will cause a drastic reduction in strength and ductility of these alloys. Depending on the shape and size of pores, significant fluctuations of strength and ductility are observed even though the sintered density of the alloys may be within 99% of theoretical density (TD).
It is an object of this invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a Wxe2x80x94Cuxe2x80x94Ni alloy having thermophysical properties suitable for use in electrical contacts and electrodes.
It is yet another object of the invention to provide a powder blend for forming a Wxe2x80x94Cuxe2x80x94Ni alloy having thermophysical properties suitable for use in electrical contacts and electrodes.
It is a further object of the invention to provide a method for direct sintering a Wxe2x80x94Cuxe2x80x94Ni alloy which substantially eliminates the formation of brittle intermetallics and slumping during sintering.
In accordance with an object of the invention, there is provided a powder blend for making a Wxe2x80x94Cuxe2x80x94Ni alloy comprising a Wxe2x80x94Cu composite powder and a nickel powder, the Wxe2x80x94Cu composite powder comprising individual particles having a tungsten phase and a copper phase wherein the tungsten phase substantially encapsulates the copper phase.
In accordance with another object of the invention, there is provided a Wxe2x80x94Cuxe2x80x94Ni alloy comprising a sintered tungsten skeleton containing a Cuxe2x80x94Ni matrix, the alloy having no brittle intermetallics.
In accordance with a further object of the invention, there is provided a method for forming a Wxe2x80x94Cuxe2x80x94Ni alloy comprising:
(a) forming a powder blend of a Wxe2x80x94Cu composite powder and a nickel powder, the Wxe2x80x94Cu composite powder comprising individual particles having a tungsten phase and a copper phase wherein the tungsten phase substantially encapsulates the copper phase;
(b) pressing the powder to form a compact; and
(c) sintering the compact to form a Wxe2x80x94Cuxe2x80x94Ni alloy.